JPS6315061B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a continuous casting method for steel in curved continuous casting equipment.

As is well known, in continuous casting, molten steel stored in a tundish is generally injected into a mold to form a slab with a predetermined cross-sectional shape, which is then continuously pulled downward and solidified to the core of the slab. After that, the steel is manufactured by cutting it to a predetermined length. While the slab is being pulled out of the continuous casting equipment, it solidifies sequentially from the surface, and is transferred to the continuous casting equipment (hereinafter referred to as
The machine length is set so that the core is completely solidified by the end of the continuous casting machine, and the drawing speed, cooling intensity, etc. are controlled.

By the way, when pouring molten steel into the mold, that is, when finishing the casting, the top part of the slab (in the present invention, the top part of the slab refers to the bottom end of the slab that is pulled out from the mold at the end of casting). It is necessary to reliably solidify the material before pulling it out. In other words, if the slab is pulled out when the top of the slab is insufficiently solidified, the solidified shell formed on the slab surface will break in the continuous caster, causing molten steel to flow out and bleed, resulting in equipment damage and damage. A situation that causes a steam explosion may occur, posing a major problem in terms of equipment and safety.

Therefore, in conventional continuous casting, the casting speed is greatly reduced toward the end of casting, and in extreme cases, it is common practice to stop drawing and allow the top of the slab to sufficiently solidify before gradually pulling it out. It was hot. For this reason, in the final stage of casting, the productivity was significantly reduced, and the temperature of the slab located in the continuous casting machine was naturally also significantly reduced.

In particular, in recent years direct rolling (hereinafter referred to as CC-DR), in which slabs produced in the continuous casting machine are immediately sent to the rolling process and rolled without cutting them, has been actively adopted. When performing DR, it is extremely important to maintain the temperature of the slab at a high temperature. Therefore, due to the temperature drop at the end of the casting process, CC
-There are many CC-DR cases where DR cannot be implemented,
This was a major obstacle to implementing CC-DR.

The present invention was developed as a result of repeated various experiments in order to fundamentally solve the problems at the final stage of casting. Forced cooling is stopped after leaving the mold until the explosive limit position of the secondary cooling zone is reached, and forced cooling is performed to perform head hardening after passing the explosion limit position and before reaching the unsolidified part outflow limit position. The present invention relates to a curved continuous casting method for steel, which is characterized by the following.

The present invention will be described in detail below based on Examples.

FIG. 1 is a cross-sectional structural diagram of a well-known general continuous casting machine. In the figure, 1 is a pot for storing molten steel 2, and 3 is a tundish. An injection nozzle 4 is attached to the tundish 3, and the molten steel 2 in the tundish 3 is injected into the mold 5 through the injection nozzle 4. 6 is a slab pulled out from the mold 5, 7 is a cooling device for cooling the slab 6, and 8 is a cooling device for continuously pulling out the slab 6 and cooling the slab 6.
a group of guide rolls that straighten and convey the slab horizontally; 9 a cutting device that cuts the slab 6 to a set length;
10 each shows a piece of steel cut to a set length.

In normal continuous casting operations, the molten steel 2 in the tundish 3 is injected into the mold 5 through the injection nozzle 4 to form a slab 6 having a predetermined cross-sectional shape, and the slab 6 is continuously cast. Then, the steel 10 is manufactured by cutting the steel 10 to a set length using a cutting device 9 provided near the end of the continuous drawing casting machine.

However, when the remaining amount of molten steel 2 in the tundish 3 becomes less than a predetermined amount or becomes zero, for example, the sliding nozzle 30 is closed and casting is completed. FIG. 2 is a perspective view showing the top portion of the slab immediately after it has been pulled out from the mold 5 after the completion of the casting.

The slab 6 is primarily cooled in the mold 5,
A solidified shell 61 of a predetermined thickness is formed on the side surface, but since the top surface 62 is covered with powder and slag, solidification does not progress sufficiently, and unsolidified portions are exposed or an extremely thin layer of solidified shell is formed. has been generated. If cooling water is injected in this state, a large amount of water vapor will be generated, which may cause an explosion.

On the other hand, in a curved continuous casting machine, the mold 5
The mold 6 that has exited the mold 6 is gradually curved from the vertical direction, and is finally straightened to the horizontal direction and then pulled out.
Therefore, if the solidified shell 61 of the top surface 62 reaches the curved part while being pulled out in a substantially vertical direction and does not reach a predetermined thickness, the top surface 61 will be damaged as shown in FIG.
is tilted with respect to the horizontal line, and unsolidified molten steel 63 flows out, both of which result in a serious accident.

In the present invention, the explosion limit position refers to the position where a solidified shell 61 of a predetermined thickness or more is formed at the top of the slab and there is no risk of explosion even if forced cooling is performed, as described above, and the outflow limit position is the same. , top surface 62
is inclined and the molten steel 63 does not flow out even if the static pressure of the molten steel 63 is applied to the solidified shell 61.

Therefore, in the present invention, forced cooling is stopped after the top part 60 of the slab leaves the mold 5 until it reaches the explosion limit position in the secondary cooling zone, and after the explosion limit position has been passed, the unsolidified part flows out. It performs head hardening in which the top surface 62 and the solidified shell 61 on the side surfaces grow to a predetermined thickness or more by forced cooling until the limit position is reached.

FIG. 4 is a chart showing an example of the results of investigating the explosion limit and unsolidified part outflow limit (hereinafter simply referred to as outflow limit) in continuous casting machines with curvature radii R of 5 m and 10.5 m. In the figure, the solid line a is the explosion limit, the broken line b is the outflow limit when the radius of curvature R is 5 m, and similarly the broken line c is the outflow limit when R is 10.5 m.

That is, Fig. 4 shows the results as a function of casting speed (m/min) and elapsed time from the meniscus (min), and if forced cooling is performed in the area below the solid line a (shaded area), an explosion will occur. This indicates a high-risk area. Therefore, when the casting speed Vc is determined under the operating conditions based on FIG. 4, the explosion limit position is set accordingly.

For example, when the casting speed Vc is 1.6 m/min, the explosion limit position is approximately 2.1 m (1.6 m/min x 1.3 min) from the meniscus. Similarly, by understanding the casting speed Vc, the outflow limit position can be determined by determining the outflow limit b,
It is set from c. When the top portion of the slab 60 reaches the explosion limit position, forced cooling of the top portion of the slab 60 is started. The forced cooling is continued until the top portion 60 of the slab reaches the outflow limit zone. Due to the forced cooling, the solidified shell 6 of the slab top portion 60
1, the slab 6 is curved as shown in FIG.
Even when the top surface 62 is inclined, it grows to a thickness that is strong enough to withstand the static pressure caused by the unsolidified molten steel 63, and the head hardening referred to in the present invention is performed.

In the present invention, the slab top portion 60 may have a length (length l from the top surface 62) that allows the top surface 62 and the solidified shell 61 on the side surfaces in the vicinity thereof to form the thickness with the above-mentioned strength. Practically speaking, it is preferable that the length be 400 to 500 mm or less, which is generally cut off and removed as cropped pieces. (A normal slab other than the slab top portion 60 is hereinafter referred to as an M piece.) Now, FIG. 5 is a configuration diagram for explaining an example of a specific configuration based on the present invention, and FIG. The figure is number 5
3 is a chart showing cooling control corresponding to the figure. That is, in FIG. 5, the secondary cooling zone 70 is composed of cooling devices 7a to 7g divided into seven parts in the strand direction, where X indicates the explosion limit position and Y indicates the outflow limit position.

When the slab top part 60 is pulled out from the mold 5 and passes through each cooling device,
As shown in FIG. 6, first, while passing through the cooling devices 7a and 7b, cooling is stopped by, for example, closing the water supply control valve 11. Slab top part 60
passes the explosion limited position X, and the cooling devices 7c and 7d
When it reaches the normal M
Forced cooling is performed with a cooling intensity higher than that of cooling the piece (hereinafter referred to as normal cooling). In the figure, A: normal cooling, B: cooling stopped, and C: when the slab passes through the top.

Next, when the slab top portion 60 reaches the cooling devices 7e, 7f, and 7g where the outflow limit position Y is located, normal cooling is resumed, and when the slab top portion 60 passes through each cooling device 7, the control valves are closed. 11 is closed and forced cooling is stopped. still,
The cooling device 7 shown in FIG. 5 has been described with its cooling control block divided into seven parts in the strand direction, but by increasing the number of divisions, the explosion limit position It is also possible to change the cooling control block depending on the outflow limit position Y. Furthermore, if the distance between the explosion limit position . In the present invention, forced cooling refers to forcibly cooling the slab top portion 60 by injecting a cooling medium such as water to the top portion 60, as described above.

Next, Figure 7 shows a width of 950 mm based on the present invention.
250mm thick low carbon Al killed steel with a bending radius of 10.5m
This figure shows the change in casting speed in an example manufactured using a continuous casting machine of 2005 in comparison with a conventional method.
S in the figure indicates the sliding nozzle is closed.

In this embodiment, the length of the continuous casting machine is 37 m, the length of the secondary cooling zone 70 is 18 m, and the cooling control block includes seven cooling devices 7a to 7g as shown in FIG.
This is an example when the casting is completed under operating conditions of a casting speed of 1.6 m/min.

Accordingly, in the present invention, when the sliding nozzle 30 is closed, slag is prevented from being drawn in, and the slab top portion 60 is moved to the explosion limit position X.
The casting speed was reduced from 1.6 m/min to 1.2 m/min in order to allow a certain amount of natural cooling to occur while reaching the desired temperature (x in Fig. 7). However, the slab top part 60
passes the explosive limit position X, the casting speed is 1.6
m/min, and forced cooling was performed at a cooling intensity of 80/min·m ² while passing through cooling devices 7c and 7d (y in FIG. 7). The slab top part 60 is the cooling device 7
After passing through points c and 7d, reaching the outflow limit position Y, and passing through cooling devices 7e, 7f, and 7g, normal cooling was performed at a cooling intensity of 40/min·m ² similar to the M piece.

As a result, as shown in Figure 8, the slab 6 can maintain an extremely high temperature of 950°C or higher (temperature at the part 40 mm from the slab end surface) even at the machine end, excluding the slab top part 60. It became. In the figure, l indicates the top of the slab. In contrast, in the conventional method,
As shown by the broken line in Fig. 7, the casting speed is drastically reduced at the end of casting, and the casting time at this low speed is long. The temperature drops significantly across the entire area of piece 6,
Therefore, the slab 6 in this area had become a CC-DR blade.

As detailed above, in the present invention, the explosion limit position The position Y is set in advance, and forced cooling is stopped until the slab top portion 60 reaches the explosion limit position X, and the slab top portion 60 passes the explosion limit position X and reaches the outflow limit position Y. The present invention makes it possible to manufacture slabs extremely safely with almost no or only a slight decrease in casting speed even in the final stages of casting. This not only improves productivity, but also prevents the temperature of the slab manufactured at the final stage of casting from decreasing, making it possible to perform CC-DR on the entire slab, except for the top part of the slab, which is removed as a cropped piece. Ta.

As described above, the effects of the present invention are extremely large.

[Brief explanation of the drawing]

Figure 1 is a sectional view of a well-known general continuous casting machine, Figure 2
The figure is a perspective view of the top of the slab, Figure 3 is a sectional view of the top of the slab, Figures 4 to 8 show embodiments of the present invention, and Figure 4 shows the explosion limit and outflow limit. Diagram, Figure 5 is a structural diagram showing the secondary cooling zone,
FIG. 6 is a chart showing the secondary cooling control situation, FIG. 7 is a chart showing changes in casting speed at the final stage of casting, and FIG. 8 is a chart showing changes in temperature of the slab corresponding to FIG. 1: pot, 2: molten steel, 3: tundish, 4:
Injection nozzle, 5: Mold, 6: Slab, 7: Cooling device, 8: Guide roll group, 9: Cutting device, 10:
Steel, 11: Control valve.

Claims (1)

[Claims] 1. At the end of casting, forced cooling of the top part of the slab is stopped from the time the top part of the slab leaves the mold until it reaches the explosion limit position in the secondary cooling zone,
A curved continuous casting method for steel, characterized in that head hardening is performed by forced cooling after passing the explosion limit position and before reaching the unsolidified part outflow limit position.